In the processing of mineral-containing ores, it is necessary to separate undesirable minerals known as gangue (e.g. Al2O3, SiO2 and TiO2) from the desired minerals in ore (e.g. iron ore). One method of accomplishing this goal is to depress the flotation of a particular mineral during the normal flotation process. In mineral flotation systems, it is common to depress the gangue materials while floating the desirable mineral or minerals. In differential or reverse flotation systems, it is common to depress the desired mineral or minerals while floating the gangue. Depression is conventionally accomplished by the use of one or more depressing agents (also known as depressants) during the flotation step. The depressant, when added to the flotation system, exerts a specific action on the material to be depressed thereby preventing it from floating. The ability of the depressant to facilitate such separation is referred to as its selectivity, i.e. a more selective depressant achieves better separation of the gangue from the desired minerals.
In a typical ore flotation scheme, the ore is ground to a size sufficiently small to liberate the desired mineral or minerals from the gangue. An additional step in the flotation process involves the removal of the ultra-fine particles by desliming. Ultra-fine particles are generally defined as those less than 5 to 10 microns in diameter. The desliming process may be accompanied by or followed by a flocculation step or some other type of settling step such as the use of a cyclone separating device. This step is followed by a flotation step wherein gangue materials are separated from the desired mineral or minerals in the presence of collectors and/or frothers.
It has been conventional in many flotation systems to use naturally derived substances such as starches, dextrins and gums as depressants. In some countries, there is a prohibition against using substances such as starch which have food value in this type of commercial application.
Starch, or causticized starch, is commonly used as a depressant in reverse iron ore flotation processes. Native starch is typically digested with sodium hydroxide or boiling water before use in such applications, see for example Tang et al. “The Acidity of Caustic Digested Starch and Its Role in Starch Adsorption on Mineral Surfaces” International Journal of Mineral Processing (2012), doi: 10.1016/j.minpro.2012.06.001. Starch produces relatively small but robust flocs which can be further upgraded by washing.
Large quantities of starch are consumed as a result of its use as a depressant in flotation processes. For example, Brazilian iron ore pellet feed production in 2010 was approximately 73,000,000 Tons, which consumed approximately 50,000 Tons of starch as the depressant. Depressant consumption is expected to increase at least 4-fold by 2017.